The present invention relates to the use of propellant feed systems in the aerospace field, and more particularly to a system and method of controlling propellant cross-feed between a booster and an orbiter of a two-stage rocket.
Increasing the thrust to weight ratio (T/W) of a rocket (or other type of launch vehicle) can result in a concomitant increase in the payload capability, or performance, of the rocket. In general, the thrust to weight ratio of the rocket may be increased by decreasing the weight of the rocket and/or increasing the thrust of the rocket. Two-stage rockets typically have two-stage propellant burn, a booster stage and a continuing (orbiter) stage. The booster stage is generally provided by a booster that includes booster engines that are connected to a booster tank using a booster feed line. Similarly, the orbiter stage is typically provided by an orbiter that includes orbiter engines that are connected to an orbiter tank using an orbiter feed line. Each engine is fed propellant for combustion from its respective tank and through its prospective feed line. During launch, the booster bums during the first, booster stage until a disconnect sequence is initiated. Then, the orbiter ignites and separates from the booster to begin the continuing stage, burning until the orbiter reaches orbit.
For the typical two-stage rocket, much of the thrust of the booster is devoted to supporting the weight of the vehicle. As the early trajectory of the rocket is nearly vertical to the Earth""s surface, the weight of the vehicle results in significant gravity losses. Increasing the T/W well above 1.0 minimizes the effect of such gravity losses until the rocket is in a flight path more nearly horizontal to the Earth""s surface. Once in a more horizontal trajectory, the thrust of the orbiter may be more effectively used to increase velocity rather than fight gravity. Additional booster engines, or larger booster engines, could increase the amount of thrust available but are generally undesirable due to the cost and additional weight of the engines.
A cross-feed system used with the two-stage rocket can significantly increase payload capacity by xe2x80x9cborrowingxe2x80x9d the orbiter engine to increase the T/W during the boost phase using propellant supplied from the booster. Conventional propellant cross-feed systems include a cross-feed line connecting the booster and orbiter feed lines. The cross-feed system allows both the booster and orbiter engines to burn simultaneously during the booster stage so as to increase thrust. The additional thrust allows the orbiter to overcome gravity losses that are incurred during the early flight period. The borrowed orbiter engine is transferred back to the orbiter near the end of the boost phase. The still-firing orbiter engine propels the orbiter back into orbit using propellant from the orbiter tanks. Advantageously, the propellant cross-feed system promotes a more efficient overall use of propellant as the more efficient orbiter engines burn for the longest time. A more efficient use of propellant reduces the amount of propellant that must be carried by the rocket, thereby reducing the weight of the rocket.
Some cross-feed systems include several motorized valves that facilitate the transition from cross-feed to continuing stage operation. In one prior art system, the cross-feed system includes a cross-feed line having a motorized closing valve and an open-latched, swing pivot valve. Also, the orbiter feed line includes a second motorized valve for controlling propellant supply to the continuing engine. The latched open swing pivot valve is on the orbiter side and when unlatched will close off to seal the cross-feed line for separation. The two motorized valves of the cross-feed system allow the engines to consume propellant from only the booster tanks during the first part of the ascent mission. However, the motorized valves must be stringently timed and sequenced to avoid significant propellant flow back to the booster and to avoid starving the orbiter engine(s) during the transition phase. Such timing or sequencing requirements may pose reliability problems. In addition, the more complex, moving parts a system has, the greater the chance of reliability problems. Reliability is a highly sought-after feature in aerospace applications.
Therefore, it would be advantageous to have a propellant cross-feed system that increases the performance of a rocket by increasing the thrust-to-weight ratio. It would be further advantageous to have a propellant cross-feed system with fewer moving or complex parts and therefore a higher reliability. It would also be advantageous, if the propellant cross-feed system minimized timing and sequencing problems.
The present invention addresses the above needs and achieves other advantages by providing a propellant cross-feed system and method for supplying a propellant which is burned by a booster and orbiter engines of a rocket. The cross-feed system includes a cross-feed line connecting a booster supply line for supplying propellant to the booster engine and an orbiter supply line for supplying propellant to the orbiter engine. The cross-feed line allows propellant to flow from the booster supply line to the orbiter supply line. Propellant flow to the orbiter supply line allows the orbiter engines to burn in parallel with the booster stage engines. Backflow of propellant from the orbiter to the booster is limited by a low pressure drop check valve that closes when pressure in the orbiter line is higher than the booster line. The flow of propellant in the orbiter line is controlled by a motorized switch-over valve that opens slowly during the process of switching over from the booster stage to the continuing stage. As the flow in the orbiter line is increased, the pressure at the orbiter end of the cross-feed line also increases, causing the low pressure drop check valve to progressively close. Advantageously then, the low pressure drop check valve responds to control of the switch-over valve, obviating the need for complex valve control and sequencing schemes.
In one embodiment, the present invention includes a two-stage rocket having a booster and an orbiter, the two-stage rocket also having a propellant cross-feed system capable of supplying propellant simultaneously to both the booster and the orbiter. The rocket includes a booster and orbiter propellant tanks each capable of containing a reservoir of propellant. The booster and orbiter engines are each capable of burning the propellant and producing thrust. The rocket further includes a booster supply line connecting in fluid communication the booster propellant tank to the booster engine. The booster supply line supplies the propellant at a flow rate to the booster engine. An orbiter supply line connects in fluid communication the orbiter propellant tank to the orbiter engine. The orbiter supply line supplies the propellant at a flow rate to the orbiter engine. The rocket also includes a cross-feed system. The cross-feed system includes a switch-over valve connected to the orbiter supply line. The switch-over valve controls the flow rate of the propellant in the orbiter supply line. The cross-feed system also includes a cross-feed line connecting the booster supply line and the orbiter supply line in fluid communication. The cross-feed line has a first pressure at its booster end and a second pressure at its orbiter end. The cross-feed line includes a low pressure drop check valve configured to begin closing as the second pressure exceeds the first pressure. An increase in the flow rate of the propellant in the orbiter supply line by the switch-over valve results in an increase of the second pressure so as to begin closing the low pressure drop valve. Closing of the low pressure drop check valve inhibits the flow of the propellant from the orbiter supply line to the booster supply line.
In another embodiment, the low pressure drop check valve is a swing check valve. The swing check valve can be oriented so that gravity closes the valve during non-operating periods. In another aspect, the swing check valve is partially counterbalanced so as to be responsive to low pressure changes. A stop may be positioned near the swing check valve so as to prevent the swing check valve from fluttering when opened. Alternatively, the low pressure drop check valve may be a flapper check valve, or some type of check valve other than a swing check valve.
In another embodiment, the switch-over valve is a motorized valve that is closed during ground operations, and remains closed until near the end of the boost phase. The motorized valve operates slowly, with a traverse time on the order of about 5 seconds. Opening of the switch-over valve is initiated by a liquid level sensor in the booster propellant tank that signals propellant is nearing depletion.
In yet another embodiment, the cross-feed line includes a pair of separation disconnect valves and a pair of redundant shutoff valves. The swing check valve includes a closing switch positioned to be activated when the swing check valve is in the closed position. The closing switch activates closure of the redundant shutoff valves. A thrust termination signal from the booster causes closure of the separation disconnect valve and physical separation of the booster from the orbiter.
The present invention has several advantages. By maximizing propellant usage, the propellant cross-feed system can result in a rocket launch vehicle weight reduction on the order of 25% for a given payload weight. Alternatively, maximizing propellant usage allows an existing rocket launch vehicle to carry a heavier payload. The cross-feed system provides a steady flow of propellant to the orbiter engine through all phases of the flight from liftoff to orbiter shutdown. The operation of the propellant cross-feed system ensures that the orbiter propellant tank is nearly full after detachment from the booster, thereby enabling efficient use of the orbiter propellant. The cross-feed system and operation are relatively simple, which contributes to the high reliability required for rocket launch vehicles. Automatic closure of the of the low pressure drop check valve in response to increasing flow through the switch-over valve eliminates the need for the complex control of sequencing and timing typically required in systems with multiple motor-operated valves. The relatively slow opening speed of the. switch-over valve minimizes waterhammer effects as the orbiter engine flow is transitioned from the booster tank to the orbiter tank. Also, inasmuch as the flow through the cross-feed line is essentially zero when the switch in the check valve activates the redundant shutoff valves, rapid closure of the redundant shutoff valves will not generate any waterhammer effects.